Wed, 31 Jan 2018 (NZDT)

While working on test/v3 for sctp.c, test-icmp.py failed.
  Noticed the following in pltmodule.c from 24 Jan 2018:
In plt_new_object:
   DataObject *d = (DataObject *)py_type->tp_alloc(py_type, 0);
   d->type = type;  d->kind = kind;  d->data = data;
   d->mom = mom;  Py_INCREF(mom);
   d->mom = mom;  if (mom == Py_None) Py_INCREF(mom);
   /* Class attribute functions are called via a _getseters struct, that
      increments Py_REFCNT for the object itself, i.e. in them 'self' has
      a borrowed reference.  We must not Py_INCREF(self) from them!
      However, most - if not all - such functions pass PyNone as mom,
      we need to Py_INCREF(PyNone)  18 Sep 2017 (NZST) */
   d->l2p = l2p;  d->l2_rem = l2_rem;

Remembered that this problem went away if d->mom was set to PyNone.
(1): the d->mom = mom; lines are redundant
(2): the second PyINCREF(mom) only increments for PyNone, which is silly!

Deleted the second d->mom line (it doesn't matter if we PyINCR(PyNone),
  it gets PyDECREFed in pltData_dealloc()
Deleted the above comment.
Checked that we never PyINCR(self) - we don't.

After all that, all 34 of the test programs work properly, on pyv2 and v3.